This invention relates to a simple and inexpensive sensor module for use in residential alarms.
Devices for sensing carbon monoxide and triggering an alarm in the presence of excessive concentrations of carbon monoxide (CO) that may be hazardous to life or health are presently available for many industrial applications, but such devices are still too costly for use in most homes. These devices may utilize electrochemical sensors, semiconductor sensors, colorimetric detectors or IR (infra-red) detectors. Typical electrochemical industrial alarms have the potential to be used in homes because of their excellent sensitivity and selectivity. The electrochemical alarms may be preferred to the semiconductor sensors because the semiconductor must be heated and therefore utilize significant power which limits battery life or requires drawing current from the residential power line (e.g., via a plug in a wall receptacle or permanent wiring of the alarm into the house). Infra-red detectors of CO are expensive and bulky, requiring a long path length, heated IR sources, and expensive detectors. Further, the present lower detection limit of semiconductor sensors may be 10 to 100 times higher than that of electrochemical sensors. Typical sensitivity ranges are 50-200 ppmv (parts per million by volume) for semiconductor sensors and 0.1-100 ppmv for electrochemical sensors. Other CO detectors, e.g., those of the colorimetric type, also lack sensitivity and/or reversibility or accuracy or convenient output to trigger an alarm.
The current trend is towards a reduction in the maximum permissible CO levels. The older limits set by the Occupational Safety and Health Administration are 50 ppmv over an eight-hour period and 400 ppmv for 15 minutes in workplace atmospheres. However, a concentration limit set by the Environmental Protection Agency is as low 9 ppmv CO for the outdoor atmosphere.
Ideally, a home should have only background CO present and this may be very low, near to 0 ppmv in the countryside or 1-2 ppmv in the city or close to combustion sources. A reading of 9 ppmv is indicative of a problem with the indoor air quality. Indeed, even a reading of 4 ppmv has been found to indicate potential problems. Therefore it is important to have a sensitive detector that will alarm at low levels.
It is therefore one object of this invention to provide an indoor hazard-warning device that will alarm at a preselected CO level in the range of 4-20 ppmv of CO in air or possibly even lower, if required. The preferred alarm level may depend on future evaluations of the health effects of CO on humans and of the levels of CO that may be indicative of other hazardous conditions, such as a smoldering fire or a leaky furnace.
It is a further object of this invention to provide such a hazard-warning device in a form that is inexpensive and adaptable for insertion into a typical residential smoke alarm. This implies that the device must be small, powered by a small battery, and drawing a current that is sufficiently low to assure longevity of the battery.
The heating that is required for the operation of a semiconductor-type sensor may pose a potential fire hazard. It is therefore another object of this invention to provide a CO-sensing device in a form that is intrinsically safe for home use. This implies that the device must not require heating or have otherwise hazardous features.
The responses of semiconductor-type or of colorimetric sensors to increased concentrations of carbon monoxide are much slower than those of electrochemical sensors. It is therefore still another object of this invention to provide a warning device that responds rapidly to the presence of elevated levels of carbon monoxide.
It is necessary to test warning devices periodically and verify that they function properly. This introduces the problem of reversibility. Colorimetric devices either do not revert to their original state after exposure to a test sample or do so very slowly. Semiconductor sensors also require a long time to reset. In contrast, electrochemical sensors revert to their zero background responses shortly after the CO concentration drops to zero. It is therefore yet a further object of this invention to provide a CO-alarming device that can be conveniently tested and thereafter quickly reset for normal operation.
Although it is clear from the foregoing discussion that electrochemical sensors offer important advantages over semiconductor-type, colorimetric, or IR detectors for CO alarms, there is still the problem of the relatively high cost and complexity of present electrochemical CO-monitoring devices. These industrial devices utilize amperometric two-electrode or, most often, three-electrode sensors that comprise a CO-sensing electrode, a counter electrode, and a reference electrode and yield a current that is proportional to the concentration of carbon monoxide in the ambient air.
To assure good accuracy and stability, the potential of the sensing electrode is kept fixed relative to that of the reference electrode by means of a potentiostat circuit that also serves to measure the sensor current. The complexity and cost of the sensor and of the potentiostat circuit presently limit the use of electrochemical CO sensors to industrial applications only. The complex circuit serves to reduce noise and provides amplification and signal conditioning for outputs to displays, relays and computers. Many circuits also need to compensate for the effects of temperature fluctuations on the signal.
It is therefore yet another object of this invention to provide an electrochemical sensor and a simplified current-measuring circuit that is inexpensive and readily adaptable for use in existing home or other residential fire alarms.